1. Field of the Invention
This invention relates to pneumatic radial tires for truck and bus mounted on 15xc2x0 drop center rims (hereinafter abbreviated as 15xc2x0 taper radial tire). More particularly it relates to a tubeless pneumatic radial tire for truck and bus adaptable to a drop center rim having a 15xc2x0 tapered bead seat (15xc2x0 drop center rim). Especially, the invention is concerned with a 15xc2x0 taper radial tire for truck and bus having advantageously improved durability of a bead portion by rationalizing an outer profile and a constituting material shape of the bead portion engaging with a flange of a rim.
2. Description of Related Art
As mentioned above, the 15xc2x0 taper radial tire for truck and bus is a tubeless (T/L) tire. In order to directly mount the tire of this type on a one-piece mold rim 21 having a section profile as shown in FIG. 20, there is applied a drop center rim (hereinafter referred to as 15xc2x0 drop center rim) in which a height of a flange 21F is made considerably lower than that of a width-wide flat base rim having a section profile shown in FIG. 21. Owing to the peculiar section profile of this flange, a bead portion of the T/L tire for truck and bus considerably differs from a bead portion of a tubed (W/T) tire to be mounted on the width-wide flat base rim. Further, the T/L tire is characterized by having a structure that a taper of about 15xc2x0 is applied to a bead base of the bead portion so as to match with a taper of 15xc2x0xc2x11xc2x0 formed on the bead seat of the rim 21 for ensuring air tightness and at the same time a given interference is applied to the bead portion.
As seen from the above description, although both the W/T tire and the T/L tire for truck and bus are used under the same heavy loading condition, a great difference in external force applied to the bead portion during the running under the heavy loading is naturally caused between both the tires. Because, the T/L tire having a lower height of the flange 21F becomes more unfavorable for fixation of the tire to the rim and the fall quantity of the bead portion toward the outside of the tire becomes larger as compared with that of the W/T tire. As the fall quantity increases, it is needless to say that problems with the bead portion are apt to be caused. Furthermore, with the W/T tire for truck and bus is difficult to attain automation in the mounting onto the rim, while the T/L tire is made possible to easily assemble on the rim by means of an automatic assembling device. As a result, the use of the T/L tire becomes more popular.
Therefore, various countermeasures as mentioned below have been taken up to the present time in order to improve the durability of bead portion in the T/L tire preventing the occurrence of bead portion trouble. These countermeasures are roughly divided into means for improving the rigidity of the bead portion and means for rationalizing the arrangement and shape of various cord layers extended from the bead portion to the sidewall portion.
As the former means for improving the rigidity, there are proposed (1) an increase of bead portion volume and additional arrangement of bead portion reinforcing cord layer, particularly means for the former volume increase in which since the bead core plays a basic role, a bead core having a low sectional height and a wide sectional width is arranged in correspondence with the low flange height of the aforementioned rim and a hard stiffener rubber extending taperedly outward in the radial direction of the tire is arranged from an outer peripheral surface of the bead core along a main body of a carcass ply to increase a volume of this rubber occupied in the bead portion as far as possible, (2) means for rendering an inner peripheral surface of a bead core having a flat and tetragonal section or a bead core having a flat and hexagonal section as mentioned in detail below into a tapered surface substantially matched with the surface of 15xc2x0 tapered bead seat to more strengthen the engagement of the bead portion with the rim, and the like.
As the latter means for rationalizing the arrangement and shape of the cord layer, there are proposed (3) means for adjusting heights of a turnup end position of a radial carcass ply and an end position of a bead portion reinforcing cord layer, or means for adjusting positions of these ends in the widthwise direction of the tire, (4) means for mitigating stress at an end portion of rubber surrounding the above ends, (5) means for rationalizing a radial carcass line (a line of thickness center of the main body of the carcass ply other than the turnup portion thereof), and the like.
All of the above means for improving the durability of bead portion have developed effects as they were. However, the above conventional means and their extensions become no longer impossible to cope with the present circumstances such as more long-lived tire performance, increase of demand for the formation of a recapped tire after the tread rubber is worn, severer demands for weight reduction and the like.
Particularly, the increase of the bead portion volume and the additional arrangement of the bead portion reinforcing cord layer are inappropriate for weight reduction. Furthermore, the excessive volume increase and additional arrangement bring about a greater amount of heat generation during the running of the tire under the heavy loading and hence the inside of the bead portion becomes a high temperature. Such a high temperature brings about the rubber deterioration and the adhesion degradation, whereby separation failure is apt to be caused from cracking at the turnup end portion of the carcass ply or the end portion of the bead portion reinforcing cord layer. Further, when the tire is used under condition of such a high temperature over a long period, the rationalization of the arrangement and shape of the cord layer with much effort is damaged by a large creep deformation of rubber as mentioned later and hence the given object can not be attained. Also, the shape of the carcass line is lost to bring about degradation of the steering stability.
Even when the increase of bead portion volume and the additional arrangement of bead portion reinforcing cord layer are within adequate ranges, the T/L radial tire for truck and bus mounted on the rim having a low flange height is naturally required to ensure a volume of the bead portion sufficiently durable to the heavy loading. When the tire having such a bead portion structure was run over a considerably longer distance such as 100,000 km, it has been confirmed that the bead portion creates a large creep (plastic) deformation as a whole. The state of this deformation is partially and sectionally shown in FIG. 22.
FIG. 22 is a section view of a main part of the bead portion when an assembly of T/L radial tire having a tire size of 11R22.5 and a rim is inflated under an internal pressure of 7 kgf/cm2 (under no load), in which a portion shown by a solid line is a new tire and a portion shown by a broken line is a tire after actual running over 100,000 km. As seen from FIG. 22, the bead portion after the running over 100,000 km creates a non-restoring creep deformation directed toward the outside of the tire.
In the tire after running (portion shown by the broken line), the hexagonal bead core causes a large shape lose and moves in a direction shown by arrows and hence the carcass ply and its turnup portion are largely pulled out in a direction shown by arrows therealong. Such a large pulling-out phenomenon always causes a large shearing strain represented by a lozenge at both sides of the turnup end portion in FIG. 22. As a result that the above large shearing strain under the inflation of internal pressure simultaneously acts together with shearing strain produced in the turnup end portion during the running under the heavy loading, a crack is first created at the turnup end portion and grows into the separation failure as the running distance becomes long. Therefore, even if it is intended to rationalize the aforementioned means for improving the durability of the bead portion, the sufficient bead portion durability can not be realized.
It is, therefore, an object of the invention to provide pneumatic radial tires for truck and bus mounted on 15xc2x0 drop center rim which can maintain adequate arrangement and shape of bead portion reinforcing cord layer inclusive of a carcass line while controlling excessive increase of bead portion volume and holding weight reduction without extra additional arrangement of the bead portion reinforcing cord layers and suppressing temperature rise in the inside of the bead portion during the running under heavy loading as far as possible, and can properly control the creep deformation of the bead portion to considerably improve the bead portion durability concerning with cracks at the turnup end portion of the carcass ply or the end portion of the cord layer and separation accompanied therewith and hold the expected excellent steering stability.
According to a first aspect of the invention, there is the provision of pneumatic radial tire for truck and bus use mounted on an approved 15xc2x0 drop center rim comprising a pair of bead portions, a pair of sidewall portions, a tread portion, at least one carcass ply extending between bead cores embedded one in each bead portions so as to reinforce these portions, the at least one carcass ply being reinforced with cords radially arranged therein and wound around the bead core from the inside of the tire toward the outside thereof to form a turnup portion in each bead portion and a main body therebetween, each of said bead cores being a wound laminate of steel wire having a round or tetragonal polygonal shape in cross section, a belt superimposed on an outer periphery of the at least one carcass ply to reinforce the tread portion and comprised of two or more cross steel cord layers and a composite side-rubber extending from a bead base of each bead portion through an outside of the bead portion to a tread rubber of the tread portion and consisting of a rubber chafer and a sidewall rubber, wherein an outer surface portion of at least the composite side-rubber among members constituting each bead portion of the tire has a curved shape concavedly directed toward the outside of the tire at a region opposite to an inner curved surface of a slantly rising portion in each flange of the approved rim.
The bead portion of the conventional 15xc2x0 taper radial tire for truck and bus use comprises a bead base having a given interference to a bead seat of the rim for holding air tightness as T/L tire when the tire is mounted onto a 15xc2x0 drop center rim and inflated under a given internal pressure, and has an outer surface portion giving a given interference to the flange of the rim. These interferences serve to strongly fix the bead portion to the rim in addition to the maintenance of air tightness. As the section of the bead portion in the conventional tire is shown in FIG. 23 together with a section profile of the rim shown by phantom line, the latter interference is attained by forming an outer surface of a composite side-rubber located at a position of contacting and engaging with the flange with an arc convexedly extending toward the outside of the tire and having a relatively large radius of curvature.
When the conventional T/L tire is mounted onto an approved rim and inflated under a high internal pressure defined in JATMA or TRA such as 8.00 kgf/cm2 as a cold inflation pressure for tire size of 11R22.5 (110, 120 PSI in TRA), the convex arc portion of the composite side-rubber is deformed along the inner curved surface of the flange (concave curved surface directing toward the outside of the tire) by such an inflated internal pressure and hence a large contact pressure is applied to such a deformed portion. In FIG. 24 is shown a distribution state of contact pressure at positions A-E of the bead portion with the rim in the conventional tire having the above tire size. The position D corresponds to a a position indicating maximum deformation quantity and is under an influence of the contact pressure considerably larger than those in the other positions. Moreover, the contact pressure distribution shown in FIG. 24 is obtained under a cold inflation pressure of 7.0 kgf/cm2, so that when the tire is actually run under loading, the internal pressure is raised to about 1.2-1.4 times the cold inflation pressure by a greater quantity of heat generation and also the outward flexing of the bead portion under loading is added. As a result the contact pressure distribution at the state of use is largely shifted toward the side of high contact pressure as compared with that shown in FIG. 24.
When the tire is run under loading, the composite side-rubber of the bead portion in a region contacting with the flange of the rim repeats such a motion that the rubber shrinks in the radial direction of the tire and along the circumferential direction of the tire at a zone of the tread contacting with ground and restores to original state at a zone kicking out from the ground contact zone. Since this motion is a relative motion to the flange of the rim, the quantity of heat generation based on hysteresis loss becomes larger at a position that the contact pressure becomes higher. A part of heat generated is released to the exterior through the rim, while the remaining greater quantity of heat generated is stored in the rubber portion known to be a poor conductor of heat and the stored heat is gradually diffused to finally render the bead portion into a high temperature.
When such a high-temperature state is held over a long period of time, the creep deformation is caused in rubber. Although the region of the bead portion other than the composite side-rubber is naturally rendered into a high temperature through heat generation through hysteresis loss being a viscoelastic property inherent to rubber based on amplitudes of internal strain and internal stress produced by the repetition of loading and its release during the running of the tire, the heat generation produced in the composite side-rubber under a high contact pressure to the flange particularly contributes to the occurrence of creep deformation. Furthermore, the composite side-rubber located under the high contact pressure prematurely causes the wearing and finally wears up to a shape extending along the flange of the rim, which is so-called rim chafing phenomenon. That is, the bead portion exposed to high temperature over a long period of time causes creep deformation to a shape shown by a broken line in FIG. 22 together with the above rim chafing.
In creep deformation, the shifting, deformation and shape loss of the beat core damage the function inherent to the bead portion in that cord layers such as carcass ply and the like are closely fixed between the bead core and the bead seat of the rim. Therefore, the fixed state of cords in the carcass ply bearing the high internal pressure is slackened and finally the pulling-out of the carcass ply as previously mentioned in FIG. 22 is apt to be caused. As a result, a large shearing strain is produced in the turnup end portion of the carcass ply to create a breaking nucleus in a boundary face between the cord and the rubber surrounding the cords, which progresses in the occurrence of failure from cracks to separation. In this point, it can be said that the shifting, deformation and shape losing of the bead core considerably and badly affect the durability of the bead portion.
The steel wire constituting the bead core has a circular or tetragonal shape at section. The former circular shape includes a complete circle and an oval shape near to the circle, while the latter tetragonal shape includes a complete tetragon as well as a lozenge, tetragon provided with four rounded corners, tetragon provided with a small protrusion and the like. Further, the wiring lamination of such a steel wire may be carried out by various shaping methods. As an example of these methods, a single long steel wire is spirally wound side by side to form an innermost peripheral portion and further spirally wound on the innermost peripheral portion side by side riding on the each wound steel cord constituting the innermost peripheral portion and then such a procedure is repeated to form a wound laminate of the steel cord. As another example, a plurality of ring-shaped wires may be gathered side by side in horizontal and vertical directions to form a wound laminate. Moreover, an inner peripheral surface of the bead core is desirable to have a taper of about 15xc2x0 with respect to a bead base line parallel in the same plane as a rim diameter line mentioned later viewing at a section of the bead core.
On the contrary, when the outer surface portion of at least composite side-rubber among members constituting the bead portion of the tire are rendered into a curved shape concavedly directing toward the outside of the tire at a region opposite to an inner curved surface of a slantly rising portion in the flange of the approved rim, the composite side-rubber contacting with the flange of the rim is possible to obtain an even contact pressure distribution to the inner curved surface of the flange under not only a high internal pressure but also a heavy loading, which controls the aforementioned creep deformation to a minimum and particularly develops the effect of effectively suppressing the shifting, deformation and shape losing of the bead core. As a result, it is possible to largely reduce the shearing strain acting to the turnup end portion of the carcass ply and the durability of the bead portion can considerably be improved.
In order to further enhance the above effect, it is effective that the concavedly curved surface of the composite side-rubber is located between a bead base line and a first straight line passing through a radially outer surface of a steel wire located at an outermost side of the bead core in the radial direction of the tire and being parallel to the bead base line and a curve appeared in the concavedly curved surface of the composite side-rubber at a section of the tire is comprised of plural arc segments smoothly connecting to each other and having a peak in one of these segments, and the peak is located between the first straight line and a second straight line passing through an innermost side of the turnup portion of the carcass ply around the bead core in the radial direction of the tire and parallel with the bead base line.
The term xe2x80x9cbead base linexe2x80x9d used herein means a straight line passing through an intersect between an extension line of the bead base and a straight line concaved with a line of an outer surface contour of a bead heel portion and perpendicular to a rotating axial center of the tire, and being parallel with the rotating axial center of the tire in the section of the tire.
In order to control creep deformation, it is practically effective that the concavedly curved surface of the composite side-rubber is formed so that a radius of curvature R1 of an arc segment having the peak among the arc segments constituting the concavedly curved surface and a radius of curvature R of a major inner curved surface in the flange at a section of the approved rim satisfy a relationship of 0.4xRxe2x89xa6R1xe2x89xa61.6xR.
Further, it is practically desirable that when a hypothetic arc convexedly extending toward the outside of the tire at section of the tire is pictured to pass through a first intersect between the first straight line among both the straight lines and an outer contour line of the bead portion and a second intersect between the bead base line and a surface contour line of the bead heel portion perpendicular to the bead base line and smoothly connect to the outer contour line in the vicinity of the first intersect, a distance D between the peak and the hypothetic arc as measured on a normal line passing through the peak toward the hypothetic arc is within a range of 22-70% of a distance d between the hypothetic arc and a position of the turnup portion of the carcass ply located on the normal line.
The tire according to the invention includes a case of using no bead portion reinforcing cord layer and a case of using a bead portion reinforcing cord layer. The former case is enough to take the constructions as mentioned above. In the latter case, at least one reinforcing cord layer is piled on the outer surface of the carcass ply in the bead portion, and the distance D between the peak and the hypothetic arc as measured on the normal line passing through the peak toward the hypothetic arc is within a range of 22-70% of a distance d between the hypothetic arc and a position of an outermost bead portion reinforcing cord layer located on the normal line, whereby the occurrence of crack and separation at the turnup end portion of the carcass ply and the end portion of the bead portion reinforcing cord layer are simultaneously controlled.
In order to further enhance the effect of concavedly curved surface of the composite side-rubber, it is effective that a convex arc segment further projecting from the hypothetic arc toward the outside of the tire and having a peak is provided on an outer surface of the bead portion at at least one of inner side and outer side of the concavedly curved surface of the composite side-rubber in the radial direction of the tire to smoothly connect to the concavedly curved surface, and a distance L between the peak and the hypothetic arc as measured on a normal line passing through the peak toward the hypothetic arc is not more than 0.65 times of the distance D.
As another means for more enhancing the effect of concavedly curved surface of the composite side-rubber, it is effective that a convex arc segment having a peak located inward from the hypothetic arc and projecting toward the outside of the tire is provided on an outer surface of the bead portion at at least one of inner side and outer side of the concavedly curved surface of the composite side-rubber in the radial direction of the tire so as to smoothly connect to the concavedly curved surface, and a distance M between the peak and the hypothetic arc as measured on a normal line passing through the peak toward the hypothetic arc is within a range of 0.1-0.5 times of the distance D.
The above two formations of the convex arc segment may be applied separately or used together.
According to a second aspect of the invention, there is the provision of pneumatic radial tire for truck and bus use mounted on an approved 15xc2x0 drop center rim comprising a pair of bead portions, a pair of sidewall portions, a tread portion, at least one carcass ply extending between bead cores embedded one in each bead portion to reinforce these portions, the at least one carcass ply being reinforced with cords radially arranged therein and wound around the bead core from the inside of the tire toward the outside thereof to form a turnup portion in each bead portion and a main body therebetween, each of said bead cores being a wound laminate of steel wire and having a round or a polygonal shape in cross-section, a belt superimposed on an outer periphery of the at least one carcass ply to reinforce the tread portion and comprised of two or more cross steel cord layers, and a composite side-rubber extending from a bead base of each bead portion through an outside of the bead portion to a tread rubber of the tread portions and consisting of a rubber chafer and a sidewall rubber, wherein an envelope surface of the bead core among members constituting each bead portion of the tire has a curved shape concavedly directed toward the outside of the tire at a region opposite to an inner curved surface of a slantly rising portion in each flange of the approved rim.
The tire according to the second aspect of the invention has an arrangement of the beat core that an envelop surface among plural envelop surfaces constituting the outer envelop line of the polygonal shape at the section of the bead core is located so as to be opposite to the inner curved surface of the flange in the approved rim. In this tire, the envelop surface for steel wires of the bead core opposite to the inner curved surface of the flange in the approved rim is rendered into a curved shape concavedly directing toward the outside of the tire, whereby it is made possible to control the deformation and shape losing of the bead core through the creep deformation and also to strongly push the carcass ply between the inner peripheral surface of the bead core and the bead seat of the rim to effectively prevent the pulling-out of the carcass ply.
The polygonal shape of the outer envelop line at the section of the bead core means a triangle or more according to geometrical definition but generally indicates a tetragon or hexagon. However, the shape of the bead core inside the tire is not necessarily maintained at an accurate polygonal shape, but is generally a shape somewhat shifting from a shape formed by connecting of line segments of n points to each other in turns. When the bead core is made from a steel wire having a circular section, an intersect between extension lines is adopted instead of the point because the point is not existent. The shaping of the bead core from the steel wire is carried out as previously mentioned. Here, the bead core having a hexagonal shape at section is explained with reference to FIG. 2. In the manufacture of the bead core 4 having a hexagonal section shown in FIG. 2, a single long steel wire 4w usually coated with rubber of a very thin thickness, for example, 0.01-0.02 mm in thickness is spirally wound side by side in a shaping mold of a concave form to form an innermost peripheral portion and thereafter the spiral winding side by side riding on each wound steel wire is repeated to form a bead core of a given shape as shown in FIG. 2. An outer envelop surface Ev of the bead core made from a group of gathered steel wires makes an envelop line of substantially an oblong hexagonal shape viewing at the section of the bead core, which is generally called as a hexagonal bead core. If such a bead core is subjected to shifting, deformation and shape loss, the hexagonal shape as a hexagonal bead core is no longer held. In FIG. 2, a straight line BLc is substantially parallel to the rim diameter line and bead base line, and an envelop surface line Ev of the innermost peripheral portion of the bead core has an inclination angle xcex1 of about 15xc2x0 with respect to the straight line BLc parallel with the rim diameter line.
In order that the envelop surface of the bead core having the concavedly curved surface practically and effectively develops the effect of controlling the creep deformation likewise the aforementioned composite side-rubber, it is favorable that at least a central region of the concavedly curved surface in the envelop surface for steel wire at the section of the bead core is comprised of an arc with a radius of curvature R2 having a curvature center located on a perpendicular bisector of a line segment connecting both ends of the central portion to each other so that the radius of curvature R2 and a radius of curvature R in an inner curved surface of a flange in an approved rim satisfy a relationship of 0.7xc3x97Rxe2x89xa6R2xe2x89xa61.2xc3x97R.
In order more enhance the effect of controlling the creep deformation by the bead core, it is advantageous that a shape of the turnup portion of the carcass ply located along the concavedly curved surface in the envelop line for steel wire at the section of the bead core is comprised of one or more arcs having a curvature center located outside the tire. Further, it is favorable that a minimum radius of curvature R3 among radii of curvature in the arcs forming the turnup portion of the carcass ply is within a range of 0.75-1.2 times of the radius of curvature R in the flange of the approved rim.
The tire using the bead core having the concavedly curved envelop surface according to the invention includes a case of using no bead portion reinforcing cord layer and a case of using a bead portion reinforcing cord layer. The former case is enough to take the constructions as mentioned above. In the latter case, at least one reinforcing cord layer is piled on the outer surface of the carcass ply in the bead portion along at least the turnup portion of the carcass ply, and has an arc-shaped portion along the arcs of the turnup portion so that a minimum radius of curvature R4 among radii of curvature forming the arc-shaped portion simultaneously satisfies relationships of R4xe2x89xa7R3 and R4xe2x89xa61.65xc3x97R3 with respect to the aforementioned minimum radius of curvature R3 and the radius of curvature R, whereby the occurrence of crack and separation at the turnup end portion of the carcass ply and the end portion of the bead portion reinforcing cord layer are simultaneously controlled.
Although the above has been described with separate formation of the concavedly curved surface in the composite side-rubber and the bead core, in order to control the creep deformation at minimum to more enhance the effectiveness of the tire according to the invention, it is desirable that both the composite side-rubber and the bead core are provided with the curved surfaces concavedly directed toward the outside of the tire on the outer surface portion of the composite side-rubber and the envelop surface for steel wire of the bead core opposite to the inner curved surface of the slantly rising portion in the flange of the approved rim.
In the tire according to the invention, in order to enhance the resistance to pulling-out of the carcass ply as far as possible, it is favorable that the tire is provided with a hard stiffener rubber tapered and extending outward from an outer periphery of the bead core along the main body of the carcass ply in the radial direction of the tire and a bead core wrapping rubber surrounding the bead core and a modulus at 100% elongation of the wrapping rubber E1 is within a range of 0.75-1.20 times of a modulus at 100% elongation of the stiffener rubber E2. The wrapping rubber has a tendency of gathering between the inner peripheral surface of the bead core and the carcass ply surrounding the bead core after the building-up of the tire through vulcanization at a high temperature under a high pressure and takes an important role for enhancing the fastening force of the carcass ply.
In order to require that the composite side-rubber having the concavedly curved surface has large resistances to creep deformation and rim chafing, it is desirable that a modulus at 100% elongation E3 of at least a rubber portion having the concavedly curved surface in the composite side-rubber is within a range of 0.68-1.15 times of the modulus at 100% elongation of the hard rubber stiffener E2.
In order to require that the steel wire of the bead core has a large resistance to creep deformation likewise the composite side-rubber, it is desirable that the steel wire constituting the bead core is provided at its periphery with a very thin coating rubber and a modulus of at 100% elongation of the coating rubber E4 is not less than 0.45 times of the modulus at 100% elongation of the hard stiffener rubber E2.